1. Field of the Invention
The present invention relates to a process for removing a diol present as an impurity in a cyclic carbonic acid ester (cyclic carbonate). The present invention further relates to a mixture of a cyclic carbonic acid ester and a chain carbonic acid ester (non-cyclic carbonate), which is obtained by the above process and contains less diols as the impurities.
2. Prior Art
Cyclic or chain carbonic acid esters have been and are used, for example, as a solvent for polymers, or a solvent for various chemical reactions, or a solvent for electrolytic solution of a condenser, cell or battery. These carbonic acid esters contain, as impurities, monohydric alcohols such as methanol, ethanol and the like, and dihydric alcohols (diols) such as ethylene glycol, diethylene glycol, propylene glycol and the like. In particular, cyclic carbonic acid esters inevitably contain, as impurities, ethylene glycol and diethylene glycol (in the case of synthesizing ethylene carbonate) and propylene glycol (in the case of synthesizing propylene carbonate), since they are generally synthesized from oxides or diols,. When a carbonic acid ester is used, for example, as a solvent for electrolytic solution of cell or battery, the diols present as impurities in the carbonic acid ester adversely affect the storage properties, etc. of the cell or battery.
Japanese Patent Application Kokai (Laid-Open) No. 74,485/1993 discloses a cyclic carbonic acid ester-containing solvent as a solvent for non-aqueous electrolytic solution, and a method for reducing the diol content in said solvent to 1,500 ppm by weight or less by subjecting said cyclic carbonic acid ester to an adsorption treatment with a molecular sieve. In the method, however, the removal of diol is not sufficient because diols generally have molecular sizes larger than the pore diameters of the adsorption sites of molecular sieve.